The objective of this study is to increase our understanding of the regulation of individual nephron glomerular filtration rates (SNGFR) by exploiting the unique renal structure in birds and the highly specialized renal structure in certain desert rodents. The avain kidney has a population of nephrons resembling reptilian nephrons which function independently of each other and do not contribute directly to the concentrating mechanism. These can cease filtering altogether under some circumstances. The avian kidney also has a population of nephrons resembling mammalian nephrons which function together in the concentrating mechanism. These nephrons do not normally cease filtering, but their filtration rates can change under some circumstances. The kidneys of certain desert rodents have juxtamedullary nephrons which are several times larger than the superficial cortical nephrons, and represent an intermediate state between the avian kidney and the more commonly studied mammalian kidneys. We are interested in three major aspects of this broad objective: 1) the more precise determination of the effects of certain factors - such as sodium, chloride, or other ions, and antidiuretic hormone - on the regulation of SNGFRs within the different nephron populations of the avian and desert rodent kidneys; 2) the intrarenal patterns of blood flow, including those of the renal portal system in the avian kidney, and their regulation by factors such as antidiuretic hormone; and 3) the morphology of vertebrate kidneys as it relates to the physiology of the countercurrent multiplier system and the effects of alterations in the function of individual nephrons on the renal concentrating and diluting mechanisms. SNGFR will be measured by the constant infusion sodium ferrocyanide technique and by micropuncture techniques which we can now apply to the avian kidney. Intrarenal patterns of blood flow will be measured by silicone injection techniques are micropressure measurements. Medullary cone electrolyte gradients will also be analyzed and related to the SNGFRs of nephrons contributing to those cones.